1. Field of the Invention
This invention relates to signal processing, more specifically to audio feedback detection and elimination.
2. Description of the Related Art
Audio feedback is a common problem in many audio amplification systems, where there is an acoustic coupling between a loudspeaker and a microphone within the same audio system. When the audio feedback is out of control or howling, the loudspeaker makes a very loud sound. This loud, highly annoying squeaking sound heard in an audio system due to a large portion of the audio being re-amplified back to the system with a gain exceeding the stability margin is often referred to as “howling.” FIG. 1 is a block diagram of an audio system 100 illustrating the different components that affect the acoustic feedback problem. The audio system 100 is to amplify the speech by a talker 102. The microphone 112 picks up the speech 132 and generates speech signal 134. The speech signal 134 is strengthened by an amplifier 120 and becomes signal 136 which drives a loudspeaker 122 to produce sound. But in addition of picking up the speech signal 132, the microphone 112 also picks up sound signal 142 from the loudspeaker 122 and noises 133 from many sources, such as an air vent 103. The signal due to sound 142 mixed into the signal 134 can make a closed loop, as shown in FIG. 2. The closed loop can cause the feed back problem. FIG. 2 illustrates the system loss and gain for a closed loop for a particular frequency for the audio system 100 of FIG. 1. The room response affects how the sound is being acoustically reflected and directed, depending on the room design and the materials used, and the speaker-microphone placements. Such response, mainly signal loss, may be represented by system loss (L-sys) 140. The system amplifier 120, among other things, increases the amplitude of the signal. It can be represented by the amplifier gain (G-amp) 140. For each frequency, when the overall system gain (L-sys+G-amp) of the closed loop is below a threshold, the audio system is stable. When the overall system gain is above the threshold or theoretical limit, the system becomes unstable and howling occurs.
The audio system response consists of the electro-acoustic components of the loudspeaker 122 and the microphone 112, and the Digital Signal Processing (DSP)/Amplification etc. of the audio system. The direct path response depends on the distance of the talker 102 relative to the microphone 112 and is typically also related to the design of the room. Examples of such systems include many large conference rooms with single or multi-zone sound reinforcement, Public Address (PA) systems, and live musical performances. Howling also occurs quite frequently in conference rooms even without sound reinforcement if the room is equipped to provide highly full-duplex conferencing. In this case, the acoustic coupling is the same, but the audio system response typically incurs some additional delays, and some nonlinear DSP processing such as echo suppression and noise cancellation.
Acoustic feedback (howling) in any audio amplification system can cause many problems. “Howling” can disrupt a lecture, irritate audience, and can even damage equipment and people's hearing. Howling typically occurs in a particular frequency where the overall system gain is above the threshold. It is found that the howling condition may vary depending on the overall environment, such as the positions of microphones, loudspeakers, the position and/or movement of the talker, the loudness of his voice, the room arrangement etc. The changing nature of howling makes it difficult to deal with.
There are various methods in the prior art attempting to eliminate the acoustic feedback or at least reduce its detrimental effect. For example, US patent application publication US2003/0210797 and its commercial embodiment dbx Advanced Feedback Suppression discloses a method. It uses Fast Fourier Transformation (FFT) to identify the frequency with the largest energy and uses polynomial interpolation to pin point the frequency if it is between the FFT bins. The frequency with the largest energy is treated as the singing frequency. Once a singing frequency is detected, the singing frequency is suppressed using notch filters. Thus the howling can be eliminated.
But most prior art methods do not provide a way to predict the occurrence of howling, i.e. predict the occurrence of howling before it actually occurs. The frequency with the largest energy in the audio spectrum is not necessarily a singing frequency. It may just be a tone that has a large energy. Prior art methods that suppress all frequencies with large energy can degrade the audio system performance even if the suppressed frequency is narrowly tailored and targeted. In many cases, random noise resonates at one or more frequency locations in the loop response. In such a case the detection is more difficult because each frequency may not always increase monotonically with time due to the non-stationary nature of the noise. An even more difficult case is when the system resonates around the peaks of the human speech spectrum (this occurs if someone talks into a microphone in an extremely high loop-gain system). The difficulty in this case lies in the discrimination between normal (but very loud speech) and speech that is about to cause howling.
Another constraint that further degrades howling detection performance is the inadequate FFT frequency resolution, which can jitter the maximum spectral energy bin to its neighboring bin. Furthermore, a detector must catch the singing frequency before the signal is clipped in the analog domain or by the A-to-D and D-to-A converters because such nonlinearity can severely corrupt the frequency analysis of the howling detector. Thus detection must occur before the signal is clipped and should be as early as possible if one wishes to eliminate the annoying squeaky loud howling tones completely.
It is desirable to have a method or apparatus to detect or predict when a howling is about to occur before it actually occurs. It is desirable to accurately predict such occurrence without too many false alarms which may degrade the overall audio system performance. A failure to properly predict necessarily means you will get the howling tone.